docs(firestore): handle non-existent document (#5853)

Calling `snapshot.data.data()` on a non-existing `DocumentSnapshot` leads to:

> The method '[]' was called on null.

I see this error pop up a lot on Stack Overflow, so this behavior is not obvious to developers. 
Explicitly including a `exists` check in this code sample should lead to move devs doing the right thing.

Author: puf

docs/firestore/usage.mdx

@@ -103,6 +103,10 @@ class GetUserName extends StatelessWidget {
         if (snapshot.hasError) {
           return Text("Something went wrong");
         }
+        
+        if (snapshot.hasData && !snapshot.data.exists) {
+          return Text("Document does not exist");
+        }
 
         if (snapshot.connectionState == ConnectionState.done) {
           Map<String, dynamic> data = snapshot.data.data();